For investigating areas of objects which are close to the object's surface it is well known in the art of NMR to use so-called surface coils. An example for such a coil is disclosed in US Patent Application Publications 2002/0089330 A1 and 2002/0084783 A1. These devices, having commercially become known under the trade name “NMR MOUSE®” utilize a U-shaped magnet system with permanent magnets. In the area of the gap between the magnet poles there exist components of the static magnetic field B0 extending parallel to the surface of the magnets defined by the front surfaces of the magnet poles. In this area between the legs, i.e. within the gap, a radio frequency coil is positioned parallel to the magnet surfaces. The field lines of the radio frequency magnetic field B1 generated by the coil have components extending perpendicular to the static magnetic field B0. The field lines of the static magnetic field B0 and the field lines of the radio frequency magnetic field B1, therefore, intersect in an area above the surface and fulfil the one condition for the excitation of nuclear magnetic resonance and for the reception of nuclear magnetic resonance signals, resp., namely B0×B1≠0. This prior art apparatus operates within a relatively low frequency range of e.g. ν0=ω0/2π=15 MHz with ω0=γB0, where γ is the so-called gyromagnetic ratio. For such a frequency range the static magnetic field B0 may be generated with permanent magnets.
If this prior art apparatus is placed on a surface of a measuring object under investigation, nuclear magnetic resonance signals may be generated and received in areas close to the surface. This method has been used for various applications like material science, the characterization of elastomers, quality control, for example in the rubber industry, explorative studies for curatorial problems and medical diagnostics.
Conventional apparatuses of this kind are characterized by their relatively limited sensitivity and their limited spatial resolution. As is generally known, the signal-to-noise ratio that may be expected in NMR measurements, depends on the number of nuclear spins contributing to the signal. In the case of the inhomogeneous magnetic fields of the present apparatus, the pulse bandwidth must, therefore, be considered as the decisive factor which, in conjunction with the spatial distribution of the static magnetic field (which is not constant in space) defines the sensitive volume (cf. Balibanu et al., J. Magn. Res., 145, (2000) pp. 246-258; Hürlimann, J. Magn. Res., 152, (2001), pp. 109-123).
The best measurements may, hence, be expected when, on the one hand, destructive interferences of the measuring signals from different sub-volumes of the sample within the sensitive volume are avoided, in which B0×B1≠0 and ω0=γB0, and, on the other hand, the bandwidth of the radio frequency pulses which is linked to the B1 intensity, is large. The B1 intensity and, likewise, the signal-to-noise ratio that may be expected within the stray field of the surface coil, depend superproportionally from the reciprocal value of the distance from the surface.
If, on the other hand, a surface area shall be measured with a high spatial resolution, e.g. in the mm range, then signals from adjacent areas must be suppressed. This may preferably be done by a spatial limitation of the radio frequency field, for example with micro coils. Insofar, the inherent inhomogeneity of the B0 field is helpful.
A common approach for the imaging detection is the realization of spatial resolution by means of additional gradients, as are also used in 2D tomographs (Casanova et al., J. Magn. Res., 163, (2003) pp. 38-45). This approach, however, requires substantial design efforts and results in complex apparatuses which are difficult to operate and are, for example, inappropriate for mobile applications.
In contrast thereto it is much simpler and more cost effective in such cases to use very small coils for achieving a high spatial resolution with a high filling factor and, hence, high sensitivity. For a series resonant circuit, the quality factor Q is proportional to ω0L/R, such that a small inductivity L seems to be of little advantage. However, in the field of NMR the quality factor Q does not really set limits at low frequencies because, first, there are known resonant circuit concepts at hand bringing L and R into a range that is acceptable for the experiment, second, there is sufficient power available, and, third, a finite pulse length is necessary for the definition of the carrier frequency ω0.
In an article “NMR microscope” published in the internet journal “spectroscopy-NOW/Resonants”, 8, (2005), John Wiley & Sons (www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,5-5-7-0-89587-ezine-0-2,00.html) a so-called NMR microscope for medical diagnostic applications is disclosed. This microscope uses a tubular magnet system being configured by a tubular direct current magnet having coils at one terminal end thereof for generating an arc-shaped constant magnetic field B0. A funnel-like radio frequency antenna is positioned within the central longitudinal opening of the magnet system. The antenna consists of a plurality of capacitive/inductive rings of stepped diameter which are arranged at an axial distance with respect to each other. The antenna is tapered in the direction towards the object under investigation. Such an antenna is also disclosed in WO 2004/083883 A1 for a measuring wavelength of 1 m, corresponding to a measuring frequency of 300 MHz. The configuration of the antenna shall effect a focussing of the radio frequency magnetic field B1 into the object under investigation which, for a spatial resolution of 10 μm shall result in an enhancement of the sensitivity by a factor of 100.
This prior art apparatus has the disadvantage that it is complex in its design and has large dimensions. It is, further, dimensioned for a radio frequency range in which the required constant magnetic field B0 may no more be generated by simple permanent magnets but an electromagnet system is required instead. Accordingly, the prior art apparatus may solely be used in a laboratory environment. Mobile field applications, in particular under confined spatial circumstances, are impossible. The conditions underlying conventional NMR within a homogeneous field B0 as disclosed in WO 2004/083883 A1 are, therefore, not transferable to the present invention.